> Hi guys
>
> Got a big program hope you can help.
>
> Does anyone know of any circuit that is able to produce a constant
> current(250mA) and voltage(about 5 to 10V).If you have it can you sent it to
> me ASAP. thanks
>
> if you have any infor please mail to me
>
> THANKS

> What do you mean?
>
> Constant current over a compliance range of 5 to 10 volts?
>
> If so, just use a current limiter.
>
> soon lee wrote:
>
> > Hi guys
> >
> > Got a big program hope you can help.
> >
> > Does anyone know of any circuit that is able to produce a constant
> > current(250mA) and voltage(about 5 to 10V).If you have it can you sent

Ok the thing is that i need to drive a IR led . according to spec it need a
constant current of 250maA and 2.5 V across it. But when i used normal
supply i only manage to get 0.6V drop and current of 6mA
Is it possible to drive the led at 250mV??
{Original Message removed}

The IR Led specs say that when you apply around 2.5V it will consume
around 250mA, or by the other side, if you apply around 250mA, it should
generate a voltage drop around 2.5V.

It is somehow easy to control a current across the IR Led, but first
lets understand what you said, that you only get a Vdrop of only 0.6V
and 6mA.

First, no IR LED emitter drops only 0.6V, some can works with voltages
low as 1.3V, and they vary from 1.5 to 4V, but no 0.6V. Problem also is
about how you managed to get the 0.6Vdrop and 6mA of current. I guess
you are using a voltage source and some resistence in series.

To test it better, use any power supply you may have around with a
resistor in series, and measure everything again.
Follows possible voltages and resistors:

+5Vdc - Use a 100 or 50 Ohms resistor in series with the LED.
+12Vdc - Use a 240 or 120 Ohms resistor in series with the LED.

Measure again the Voltage across the LED and current flowing through it,
and report back here, so we can help you better.

By the way, the current in a IR led is not critical, so, if the specs
say 250mA (this is a bit high for a IR led), lets imagine that you have
+5Vdc to feed it, so, 5V - 2.5 (Vdrop) will result in 2.5V over a
current limiter resistor, 2.5V / 250mA = 10 Ohms. So, connecting a 10
Ohms in series with the LED and connectig it directly to +5Vdc will
result in what you want. Are you sure about the 250mA?

Wagner.

soon lee wrote:
>
> Ok the thing is that i need to drive a IR led . according to spec it need a
> constant current of 250maA and 2.5 V across it. But when i used normal
> supply i only manage to get 0.6V drop and current of 6mA
> Is it possible to drive the led at 250mV??
> {Original Message removed}

At 09:36 PM 11/27/99 +0800, you wrote:
>Hi guys
>
>Got a big program hope you can help.
>
>Does anyone know of any circuit that is able to produce a constant
>current(250mA) and voltage(about 5 to 10V).If you have it can you sent it to
>me ASAP. thanks
>
>if you have any infor please mail to me

Take a look at just about any data sheet for a 3-pin regulator--should be a
circuit there for constant current. All you need is the regulator and one
resistor to set the current. LM317's work fairly well and can handle the
current. You will lose about 1.25 volts across the resistor. AND be sure
to put a 1 uF cap across the resistor as well--really does wonders for
current stability. DO NOT put a cap from the output to ground, however.

>At 09:36 PM 11/27/99 +0800, you wrote:
>>Hi guys
>>
>>Got a big program hope you can help.
>>
>>Does anyone know of any circuit that is able to produce a constant
>>current(250mA) and voltage(about 5 to 10V).If you have it can you sent it to
>>me ASAP. thanks
>>
>>if you have any infor please mail to me
>
>Take a look at just about any data sheet for a 3-pin regulator--should be a
>circuit there for constant current. All you need is the regulator and one
>resistor to set the current. LM317's work fairly well and can handle the
>current. You will lose about 1.25 volts across the resistor. AND be sure
>to put a 1 uF cap across the resistor as well--really does wonders for
>current stability. DO NOT put a cap from the output to ground, however.
>
>Kelly
>
>William K. Borsum, P.E. -- OEM Dataloggers and Instrumentation Systems
><.....borsumKILLspam.....dascor.com> & <http://www.dascor.com>San Diego, California, USA

Use an ordinary 7805 - it makes a regulated 5 volts, and if you put a
constant load across that, such as a lamp, then the regulator looks to
the outside world like a constant current regulator. Great for battery
charging!

> Use an ordinary 7805 - it makes a regulated 5 volts, and if you put a
> constant load across that, such as a lamp, then the regulator looks to
> the outside world like a constant current regulator. Great for battery
> charging!

I used this with a resistive load, to test power packs from Taiwan. Their
spec says so much voltage at so much current, so a constant current load and
a voltmeter made for a great tester.

I wouldn't reccomend a lamp as a load though, their resistance is
non-linear, and highly temperature dependent, and has long term drift as the
lamp ages.. A resistor is utterly predictable.

There are some fun tricks you can do with three terminal regulators that
aren't in any books. Think about how the reg works. The output V is
constant into a varying load, so the reg has to present a variable impedance
to the supply. Your current waveform is therefore present on the input, but
the input voltage waveform (if any) is not present on the output. This also
underscores the need for a good input cap, if you have anything else that
feeds from the same unreg source.